1. Field of the Invention
The present invention relates to a manufacturing method of a pneumatic tire, and more particularly to a manufacturing method of a pneumatic tire in which a belt reinforcing layer is constituted by a spiral belt.
2. Description of the Related Art
Conventionally, in a pneumatic tire, in order to reinforce a carcass with a so-called hoop effect, a belt layer and a belt reinforcing layer are arranged in an outer periphery of the carcass. Generally, the belt layer is formed by winding a first belt ply around an outer peripheral surface of a belt drum so as to be formed as a tubular shape, and thereafter winding a second belt ply around an outer periphery thereof so as to be formed as a tubular shape. The belt plies include cords which are inclined with respect to a tire circumferential direction, for example, at an angle between 10 and 40 degrees, and are laminated in such a manner that the cords intersect with each other between the plies.
Further, the belt reinforcing layer is constituted by a reinforcing ply including a reinforcing cord which is substantially in parallel to the tire circumferential direction, and there is a case that the belt reinforcing layer is formed by winding a sheet-like reinforcing ply around an outer periphery of the belt layer. However, for the purpose of increasing a high-speed durability or the like, there has been known a method of spirally winding a rubber-coated long reinforcing cord around an outer periphery of the belt layer so as to be formed as a so-called spiral belt (for example, Japanese Unexamined Patent Publication Nos. 2001-310604 and 2005-75037).
A tread rubber is attached on the outer periphery of the belt reinforcing layer so as to be integrated, and a tubular belt and tread assembly is formed. The assembly is transferred to another forming drum, and is retained in an outer peripheral side of a cylindrical member obtained by attaching an inner liner, a carcass, a bead core, a bead filler, a side wall and the like formed on the forming drum. Further, a green tire obtained by integrating the cylindrical member and the belt and tread assembly is formed by applying an internal pressure to the cylindrical member so as to be evaginated and deformed and bringing the carcass into pressure contact with an inner peripheral surface of the belt layer.
The formed green tire is set to a vulcanization forming mold, and is vulcanization formed in a state in which a tread portion is brought into pressure contact with an inner surface of the forming mold. At this time, since the inner surface of the forming mold is formed as a curved shape which protrudes to an outer side in a diametrical direction, the tread portion of the green tire is deformed in such a manner that a center portion is diametrically expanded, and the belt layer and the belt reinforcing layer are accordingly expansion deformed. However, there is a case that the belt layer generates a local deformation caused by the cord structure mentioned above. If the cord arrangement is disturbed thereby, there is a case that a dispersion is generated in a rigidity of the tread portion and a tire performance is deteriorated.
Accordingly, there has been proposed a structure in which the belt layer is formed by a drum-shaped belt drum having an outer peripheral surface protruding to an outer side in a diametrical direction, in place of a normal belt drum having an outer peripheral surface being in parallel in an axial direction (for example, Japanese Unexamined Patent Publication Nos. 2002-18977 and 2003-225953). Therefore, it is possible to previously apply a curvature to the belt layer, and it is possible to prevent the belt layer from generating a local deformation at a time of the vulcanization forming.
However, in the case of using the belt drum having the curvature mentioned above, an inclination becomes large particularly near an end portion in a width direction of the belt layer. Accordingly, there is a problem that the reinforcing cord falls down to an outer side and can not be accurately wound, at a time of forming the spiral belt as described in Japanese Unexamined Patent Publication No. 2002-18977. As a countermeasure thereof, there can be considered to arrange the end portion in the width direction of the spiral belt in an inner side than the belt layer as described in Japanese Unexamined Patent Publication No. 2003-225953, however, in this case, a constraint force of the end portion in the width direction of the belt layer is reduced, and there is generated a problem that a reinforcing effect by the belt reinforcing layer can not be sufficiently obtained.